With the progress of office automation, the demand for electrophotography-based copying machines and printers has rapidly increased and performance requirements for such equipment have become higher. There is a generally used method which comprises forming an electrostatic latent image on the photo-sensitive material, then developing the image using a toner, transferring the toner image onto a fixing sheet such as paper or the like, and heat-pressing the transferred toner image using a heat roll (a heat roll fixing method). In this heat roll fixing method, in order to achieve enhanced economical efficiency in power consumption and increased photography speed, and to prevent papers from being curled, a toner superior in fixing properties is needed, which can be fixed at a lower temperature. On the other hand, there is such a problem, a so-called offset phenomenon, in the heat roll fixing method that since a toner is contacted with the surface of the heat roll in the melt state of the toner, the toner is transferred by adhering on the surface of the heat roll, and the transferred toner is transferred again on the next fixing sheet to be fixed to contaminate it. One of the important performance requirements for a toner is not to cause this offset phenomenon as well. Meanwhile, as copiers and printers have come to employ a higher speed, high performance requirements in electrified portions have also increased. That is, a higher durability is required for a toner and a longer-term stability in printing is needed.
Furthermore, in recent years, an increase in population has increased the amount of energy and has depleted resources. In connection therewith, resources saving, energy saving, recycling of resources, etc. have been claimed. As to PET bottles, individual municipalities have started a recycling activity and have started to utilize PET bottles as clothes or containers. In addition, utilization of recycled PET is highly desired.
In the conventional technique, there has been proposed a method to improve the molecular weight and molecular weight distribution of a binder resin for a toner. Concretely, it has been tested to have a binder resin of a low-molecular weight and to lower the fixing temperature. However, the melting point has been lowered owing to the lowered molecular weight whereas the cohesive force of a resin has also been deteriorated simultaneously. Therefore, an offset phenomenon occurs to the fixing roll. In order to prevent such a problem, it has been attempted to use a binder resin having a wide molecular weight distribution by mixing a high molecular weight resin and a low-molecular weight resin, or, further to crosslink the high molecular weight portion of the binder resin or the like. With this method, however, it is difficult to satisfy the fixing properties owing to the increased viscosity of the resin.
As a binder resin for a toner using the above method, in general, a styrene acrylic resin (e.g., JP1980-6895B and JP1988-32180B, U.S. Pat. No. 5,084,368, etc), a polyester resin (e.g., JP1986-284771A, JP1987-291668A, JP1995-101318B, U.S. Pat. No. 4,833,057, etc.), or a polyol resin (e.g., JP1999-189647A, etc.) is mainly used.
However, the toners obtained by these techniques cannot be fully effective in high-speed copiers or small copiers in which the amount of heat from the heat fixing roll is not sufficiently transmitted. That is, when a polymer having a high weight-average molecular weight or a crosslinked polymer is used in order to prevent the offset phenomenon, the viscosity of a resin increases so that the fixing properties are deteriorated.
In order to achieve these requirements, there is a method to add paraffin waxes, low-molecular weight polyolefin or the like as a mold release agent to a toner. In JP1974-65232A, JP1975-28840A, JP1975-81342A or the like is disclosed a technique to use a styrene type binder resin. However, antiblocking properties and the development properties have been deteriorated whereas an offset phenomenon has been improved. It is also confirmed that the effects of a mold release agent are small when applied to a polyester resin and the developing agent is rapidly depleted when its amount is increased.
It has also been tested to improve toner performance by employing a special wax, which has been reported in JP1984-174853A, JP1998-186722A, JP2000-1532A or the like. However, a toner having much higher performance has been required from the market.
In order to provide a developing agent having sufficient fixing properties and offset resistance, and being capable of providing high image quality of copying images, it needs to give sufficient electrophotographic features to the developing agent. Many methods have been attempted heretofore in order to achieve high image quality and high resolution of copying images; however, a toner capable of overcoming the above defects and responding to the market demand could not be obtained heretofore.